Separate out ArgumentContextUtils.normalizeDoubleQuoted()

Separate out ArgumentContextUtils.normalizeDoubleQuoted()

appendString() is really just a lexer-to-logic dispatch method,
which we may end up moving. Even if not, it provides benefits
in being small and well auditable, as well as being readily
inlineable.

A nice side-effect here is that we get the control over both
whitespace and unescaping, which makes it obvious we can check
for existing escapes and then go into dealing with them, with
a known position of the first backslash.

JIRA: YANGTOOLS-1079
Change-Id: I157795634eb6caea10a819feb4a50fb9b394b267
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>